Single-top quark cross-section measurements in ATLAS

TL;DR

This paper reports comprehensive measurements of all three single top-quark production channels at the LHC, including new results at 13 TeV and evidence for the s-channel at 8 TeV, advancing understanding of top-quark production.

Contribution

It provides the first measurements of single top-quark production in all channels at 13 TeV and evidence for the s-channel at 8 TeV, expanding experimental knowledge.

Findings

First 13 TeV measurements of t-channel production

First evidence of s-channel production at 8 TeV

Detailed cross-section results for all channels

Abstract

This article presents measurements of all three single top-quark production channels. Detailed measurements of $t$-channel single top-quark production using data collected by the ATLAS experiment in proton--proton collisions at a centre-of-mass energy of 8 TeV are shown as well es first results using 13 TeV at the LHC. The associated production of a top quark and a $W$ boson is presented for data collected at 13 TeV, while the first evidence of single top-quark production in the $s$-channel is shown for data collected at 8 TeV.

Figures

Full text

SNSN-323-63

Single-top quark cross-section measurements in ATLAS

Dominic Hirschbühl111email: dominic.hirschbuehl@cern.ch

on behalf of the ATLAS collaboration

Bergische Universität Wuppertal, 42119 Wuppertal, Germany

This article presents measurements of all three single top-quark production channels. Detailed measurements of $t$-channel single top-quark production using data collected by the ATLAS experiment in proton–proton collisions at a centre-of-mass energy of 8 TeV are shown as well es first results using 13 TeV at the LHC. The associated production of a top quark and a $W$ boson is presented for data collected at 13 TeV, while the first evidence of single top-quark production in the $s$-channel is shown for data collected at 8 TeV.

PRESENTED AT

$9^{th}$ International Workshop on Top Quark Physics

Olomouc, Czech Republic, September 19–23, 2016

1 Introduction

At leading order (LO) in perturbation theory, single top-quark production is described by three subprocesses that are distinguished by the virtuality of the exchanged $W$ boson. The dominant process is the $t$-channel exchange, where a light quark from one of the colliding protons interacts with a $b$-quark from another proton by exchanging a virtual $W$ boson. The total inclusive cross-sections of top-quark and top-antiquark production in the $t$-channel in proton–proton $pp$ collisions at a centre-of-mass energy $\sqrt{s}=8$ TeV are predicted to be $\sigma(tq)=54.9^{+2.3}_{-1.9}$ pb proton–proton $pp$ and $\sigma(\overline{t}q)=29.7^{+1.7}_{-1.5}$ pb for top-antiquark production and at $\sqrt{s}=13$ TeV to be $\sigma(tq)=136.0^{+5.4}_{-4.6}\;$pb and $\sigma(\overline{t}q)=81.0^{+4.1}_{-3.6}\;$pb at next-to-leading order (NLO) precision in QCD [1, 2]. The second highest production cross section is predicted for the associated production of a $W$ boson and a top quark ($Wt$). The cross-section of the $Wt$ channel at NLO with next-to-next-to-leading logarithmic soft-gluon corrections is calculated as $\sigma(Wt)=71.7\pm 3.8\;$pb [3] for $\sqrt{s}=13$ TeV. The $s$-channel production of a top quark and a $b$-quark ($t\overline{b}$) for $\sqrt{s}=8$ yields the smallest cross section of $\sigma(s)=5.61\pm 0.22$ pb in NLO QCD [4].

In the following, measurements of all three production channels using data collected by the ATLAS experiment [6] in $pp$ collisions either at $\sqrt{s}=8$ TeV corresponding to an integrated luminosity of 20.2 fb*-1* or $\sqrt{s}=13$ TeV at the LHC corresponding to an integrated luminosity of 3.2 fb*-1* are presented.

2 $s$-channel single top-quark production

The analysis for $s$-channel single top-quark production is performed using collision data at $\sqrt{s}=8$ TeV. Events are selected if they have either of an electron or muon, two jets, where both have to be identified as a jet containing $b$-hadrons ($b$-tagged jet) and large missing transverse momentum $E_{\mathrm{T}}^{\mathrm{miss}}$. After the preselection, the main background are top-quark pair-production ($t\overline{t}$) and $W$+jets production. In order to separate the signal from the large background contributions, a matrix element method discriminant is used, see Fig. 1. The signal is extracted from the data utilising a profile likelihood fit, which leads to a measured cross-section of $\sigma(s)=4.8\pm 0.8\mathrm{(stat)}^{+1.6}_{-1.3}\mathrm{(syst)}$ pb [7]. Dominating uncertainties are Monte Carlos (MC) statistics, jet energy resolution, and the modelling of the $t$-channel single top-quark process. The result, which is in agreement with the SM prediction, corresponds to an observed significance of 3.2 standard deviations.

3 Associated Wt production

The inclusive cross-section for the associated production of a $W$ boson and top quark is measured using dilepton events with at least one $b$-tagged jet. Events are separated into signal and control regions based on the number of selected jets and $b$-tagged jets, and the $Wt$ signal is separated from the $t\overline{t}$ background using a boosted decision tree (BDT) discriminant, shown in Fig. 2 The cross-section is extracted by fitting templates to the BDT output distribution, and is measured to be $\sigma(Wt)=94\pm 10\mathrm{(stat)}^{+28}_{-23}\mathrm{(syst)}$ pb [8]. Main uncertainties are coming from the jet energy scale (JES) and the modelling of the top-quark processes.

4 $t$-channel single top-quark production

The experimental signature of $t$-channel single top-quark candidate events is given by one charged lepton (electron or muon), large $E_{\mathrm{T}}^{\mathrm{miss}}$ and two jets. Exactly one of the two jets is required to be $b$-tagged. The main background contributions are the $t\overline{t}$ and $W$+jets processes. In order to separate signal from background an artificial neural network is used, shown in Fig. 3

For the $\sqrt{s}=8$ TeV data; the total and fiducial cross-sections are measured for both top quark and top antiquark production. The fiducial cross-section is measured with a precision of 5.8% (top quark) and 7.8% (top antiquark), respectively [9]. A comparison with different MC generator setups is shown in Fig. 4 for the extrapolated total cross section.

In addition, the cross-section ratio of top-quark to top-antiquark production is measured, resulting in a precise value to compare with predictions, $R_{t}=\frac{\sigma_{t}}{\sigma_{\overline{t}}}=1.72\pm 0.09$ and presented in Fig. 5(left). Dominant uncertainties for these measurements are the JES and modelling of the top-quark processes. The total cross-section is used to extract the $Wtb$ coupling: $f_{\mathrm{LV}}\cdot|V_{tb}|=1.029\pm 0.048$, which corresponds to $|V_{tb}|>0.92$ at the 95% confidence level, when assuming $f_{\mathrm{LV}}=1$ and restricting the range of $|V_{tb}|$ to the interval $[0,1]$.

Requiring a high value of the neural-network discriminant leads to relatively pure $t$-channel samples, which are used to measure differential cross-sections. Differential cross-sections as a function of the transverse momentum and absolute value of the rapidity of the top quark, the top antiquark, as well as the accompanying jet from the $t$-channel scattering are measured at particle level and parton level. All measurements are compared to different Monte Carlo predictions as well as to fixed-order QCD calculations where these are available. The SM predictions provide good descriptions of the data.

For the $\sqrt{s}=13$ TeV data; the the total cross-sections for both top quark and top antiquark production are measured to be $\sigma(tq)=156\pm 28~{}\mathrm{pb}$ and $\sigma(\overline{t}q)=91\pm 19~{}\mathrm{pb}$, respectively [10].

The cross-section ratio is found to be $R_{t}=1.72\pm 0.20$ and compared with predictions from different PDF groups in Fig. 5 (right). The coupling at the $Wtb$ vertex is determined to be $\mathrm{LV}\cdot|V_{tb}|=1.07\pm 0.09$ and a lower limit on the CKM matrix element is set, giving $|V_{tb}|>0.84$ at the 95% CL. These measurements are dominated by systematic uncertainties, from which the uncertainties connected with MC generators are the biggest ones.